The terror group has had Europe in its sights for up to four years, it is feared, and sent dozens of jihadists back to carry out lone-wolf attacks.

“It served to put all of our agencies on edge. Just like a smoke screen, it allowed them to calmly prepare,” said Marc Trevidic, who retired as France’s chief counter-terrorism judge last year.

The offensive was orchestrated by Abdelhamid Abaaoud, the Belgian mastermind behind the Paris atrocity in November, which left 130 dead.

Abdelhamid Abaaoud was the mastermind behind the Paris attacks in November

Abaaoud, who was killed in the weeks after the attacks, is feared to have been at the head of a 90-strong cell of jihadists spread across Europe.

It has also emerged he has persuaded some fanatics to return to attack Europe by convincing them it is better that facing the horrors of the war in Syria, according to files seen by the New York Times.

Prior to his death, he was a senior figure in a unit within Islamic State in Iraq and Levant, tasked with directing attacks on European soil.

Fighters are sent back and told to decide their own targets so limit the chances of plots being traced back to the unit.